Sputter deposited thin-film resistors having low temperature coefficient of resistance (TCR) are required for the production of passive electronic components and various types of integrated circuits (ICs). Metal and alloy thin films such as Ta, NiCr, and CuNi are widely employed for relatively low value resistors with sheet resistance in the range of 20-200Ω/□ (ohms per square). Metal silicide films such as WSix and CrSi2 provide higher values of resistance to a few kilo-ohms per square but their TCR is too high to be employed in precision circuitry.
Cermet materials comprising solid solutions of metal particles in a ceramic (dielectric or semiconductor) matrix can exhibit electrical conduction by electron tunneling between the metal particles, and thus offer a wide range of resistances based on the amount of metal particles. The mechanism which control or alter the thermostability of cermet resistors is not completely understood. It has been observed that various semiconducting oxides exert an influence on the temperature response of resistivity of cermet resistors so as to make them more thermally stable. For example, TCR of these cermet films is also dependent on the compositions, and thus current thin film cermet resistors have resistance and TCR coupled through their compositions, with optimization for resistance can lead to a specific composition that is detrimental to TCR, and vice versa.
In general, only resistance or TCR for a cermet resistor can be optimized through composition engineering. For example, high resistances up to 20 kΩ/□ can be obtained in cermet films having a low percentage of metal particles, but these cermet films are often accompanied with high negative values of TCR, for example, TCR ranging from −1500 to 500 ppm/° C. for a range of resistance values from 0.001 to 0.1 Ω-cm. Alternatively, cermet films with low TCR (e.g., close to zero ppm/° C.) can be achieved by balancing the amount of their metal and ceramic components, but these cermet films have a certain range of resistances, typically less than few hundreds Ω/□).
Geometry approach can be used to increase film resistance, such as reducing the resistor film thickness or increasing the resistor path length. However, the geometry of the thin film resistors can have limitations, such as size constraints and fabrication problems such as stability and uniformity of the film properties.